1. Field of the Invention
The present invention relates to an optical scanning device for deflection scanning with light, and more particularly to an optical scanning device wherein a mirror is fixed to and driven by a reversible rotary motor of which the rotating angle of the rotary vibrator is limited by the restituting force of a torsion spring rod functioning against the driving force of the motor.
2. Description of the Prior Art
In the prior art there are already known optical scanning devices for deflection scanning of light modulated with information signals for the purpose of recording such signals on a scanned plane having a photosensitive member or reading information from such plane.
One such optical scanning device comprises a rotary vibrator composed of a torsion spring rod fixed at one end thereof to the motor body and provided at the other end thereof with a mirror, with an armature made of a material of a high magnetic permeability and with a speed detecting element, and a stator provided with a permanent magnet, a drive coil and a speed detecting coil to constitute a motor magnetic circuit and a speed detecting magnetic circuit of the reversible rotary motor. The drive coil of the stator is supplied with a sinusoidal current to generate a drive torque proportional to said current thereby causing a self resonance in a vibrating system defined by the inertial force, attenuating force and restituting force of said vibrator at a frequency specific thereto. The signals generated by said speed detecting magnetic circuit and representing the phase and amplitude of the vibration of said vibrator are supplied to a control circuit to supply a controlled voltage to said stator drive coil thereby stabilizing the vibrating amplitude of said vibrator. Such optical scanning device is disclosed for example in the U.S. Pat. Nos. 3,609,485, 3,624,574 and 3,921,045.
In the optical scanning device of the above-mentioned type, when the vibrator is subjected to a forced vibration by a sinusoidal current of a frequency .omega.=2.pi.f with a drive torque T (=F. sin .omega.t), the amplitude of said vibration is approximately proportional to the drive torque or the drive current. In order to obtain a large amplitude with a small drive current, the vibration is conducted in a so-called resonance state wherein the drive frequency .omega. is equal to the proper vibration frequency .omega..sub.n of the vibrator. In such case, the frequency .omega..sub.R, amplitude .theta..sub.R and sharpness Q of the resonance can be represented by the following equations under the condition that the attenuation constant .gamma. of the vibrating system is very small (&lt;&lt;1): ##EQU1## wherein m is the inertial moment of the vibrator, K is the suspension stiffness (spring constant) of the torsion spring rod constituting the vibrator, d and l are respectively the diameter and length of the spring portion of said torsion spring rod, G is the transversal elasticity coefficient of the material constituting said torsion spring rod, and .delta. is a so-called attenuation ratio which is a function of temperature as shown in the equation (3).
In the designing of a vibrator of the above-mentioned optical scanning device, at first the suspension stiffness K is determined from the required vibrating amplitude in consideration of the material to be employed for the torsion spring rod and the shearing stress generated therein during the vibration, and is utilized for calculating the dimensions d and l of the torsion spring rod according to the equation (1) and also for determining the inertial moment m of the vibrator as a function of the required resonant frequency .omega..sub.R. The vibrator designed in this manner generally assumes a structure of a thin oblong torsion spring rod provided at the upper end thereof with the mirror of a relatively large inertial moment, the speed detecting element and the armature. The vibrator of such structure, when assembled in the motor, tends to incline at the upper end by the magnetic attraction of the magnetic circuit, and is therefore unable to provide a stable oscillation or unable to provide a sufficient vibration accuracy of the mirror even if the vibration can be stabilized. Besides, the vibrator is characterized by the drawback of being susceptible to an external vibration in a direction perpendicular to the axis of the vibrator.
Also the torsion spring rod, which is generally composed of a resilient material such as spring steel, stainless steel or beryllium steel, changes the dimensions (d and l) thereof and the transversal elasticity coefficient G as a function of circumferential temperature, thus resulting in a change in the resonant frequency .omega..sub.R and the resonant amplitude .theta..sub.R as will be understood from the equations (1) and (2). In case of a vibrator with a resonant frequency of for example 1 KHz, said change in frequency in response to a temperature rise of ca. 20.degree. C. from the normal temperature amounts to 1 to 3 Hz, which is by no means negligibly small in high-precision scanning for example required in a laser recording apparatus.
The equations (2) and (3) indicate that the vibrating amplitude .theta..sub.R under a constant sinusoidal current in the stator drive coil is proportional to the sharpness Q of the resonance, which can be made larger for a larger value of the suspension stiffness K of the torsion spring rod or of the inertial moment of the vibrator, and for a smaller value of attenuation coefficient .gamma. of the vibrating system. Consequently, in the aforementioned optical scanning device, the resonance sharpness Q can be improved by reducing the attenuation coefficient as far as possible.